Microstructure and properties of B + C + N ternary hardening layers on Q235 low-carbon steel prepared by plasma electrolysis

Abstract

Ternary surface hardening layers were prepared on the Q235 low-carbon steel by plasma electrolytic borocarbonitriding(PEB/C/N) under constant voltage of 260–300 V for 30 min. The microstructure, phase components and hardness depth profiles of the B + C + N ternary hardening layers were examined. The ternary hardening layers were composed of a boride layer with single Fe2B phase and a transition layer. The thickness of modified boride layers on the PEB/C/N samples was about 10 μm, 15 μm and 22 μm at 260 V, 280 V and 300 V, respectively. The maximum hardness of boride layer was 2400 HV for the 300 V PEB/C/N sample. The surface free energy of bare Q235 steel and PEB/C/N samples at 260, 280 and 300 V were 41.084, 32.039, 26.906 and 25.726 mJ/m2, which were calculated on the basis of their contact angles for water and n-hexadecane. Furthermore, the hardening layers exhibited excellent corrosion behavior due to the improved hydrophobicity for the dense boride layer. The lowest wear rate of PEB/C/N sample at 300 V is about 2.0375 × 10−6 mm3/ N·m, which is only 1/15 of the bare Q235 steel, the ternary hardening treatment shows a better wear resistance than the bare Q235 steel.